The lower airway consists of the lungs and the air tubing (bronchi) that supplies them. The lungs have some very interesting and unique protective mechanisms that put forth a great effort to prevent infection. Obviously, the air we breathe is not sterile and contains many contaminants such as dirt, dust, pollen, chemicals, and particles of a million other things as well as bacteria, viruses, and fungal elements.

A horse with pleuritis and indwelling drains into the thoracic cavity allowing for continuous drainage of inflammatory material.

The protection actually starts in the upper airway with filtering, humidifying, and warming of the inspired air. The upper respiratory system, trachea, and bronchi are lined with tissue that always is covered with a wet/sticky mucus to which contaminants in the air will stick. To take the process one step further, the tissue lining the trachea and bronchi has a billion or so cells with extremely small hair-like fibers sticking out into the airway (these hair-like fibers are called cilia)